The present invention relates to magnetic recording media, such as thin film magnetic recording disks, and to a method of manufacturing the media. The present invention has particular applicable to high areal density longitudinal magnetic recording media exhibiting low noise and enhanced magnetic performance. cl BACKGROUND ART
Magnetic recording media are extensively employed in the computer industry and can be locally magnetized by a write transducer or write head to record and store information. The write transducer creates a highly concentrated magnetic field which alternates direction based upon bits of the information being stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the recording medium, grains of the recording medium at that location are magnetized. The grains retain their magnetization after the magnetic field produced by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the recording medium can subsequently produce an electrical response to a read sensor, allowing the stored information to be read.
There is an ever increasing demand for magnetic recording media with higher storage capacity, lower noise and lower costs. Efforts, therefore, have been made to reduce the size required to magnetically record bits of information, while maintaining the integrity of the information as size is decreased. The space necessary to record information in magnetic recording media depends upon the size of transitions between oppositely magnetized areas. It is, therefore, desirable to produce magnetic recording media that will support the smallest transition size possible. However, the signal output from the transition must avoid excessive noise to reliably maintain the integrity of the stored information. Media noise is generally characterized as the sharpness of a signal on readback against the sharpness of a signal on writing and is generally expressed in signal-to-media noise ratio (SMNR).
The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of remanent coercivity (Hr), magnetic remanance (Mr), coercivity squareness (S*), SMNR, and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements.
The linear recording density can be increased by increasing the Hr of the magnetic recording medium, and can be accomplished by decreasing the medium noise, as by maintaining very fine magnetically non-coupled grains. Medium noise in thin films is a dominant factor restricting increased recording density of high density magnetic hard disk drives, and is attributed primarily to inhomogeneous and large grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
Longitudinal magnetic recording media containing cobalt (Co) or a Co-based alloy magnetic films with a chromium (Cr) or Cr alloy underlayer deposited on a non-magnetic substrate have become the industry standard. For thin film longitudinal magnetic recording media, the desired crystallized structure of the Co and Co alloys is hexagonal close packed (HCP) with uniaxial crystalline anisotropy and a magnetization easy direction along the c-axis is in the plane of the film. The better the in-plane c-axis crystallographic texture, the more suitable is the Co alloy thin film for use in longitudinal recording to achieve high remanance. For very small grain sizes coercivity increases with increased grain size. The large grains, however, result in greater noise. Accordingly, there is a need to achieve high coercivities without the increase in noise associated with large grains. In order to achieve low noise magnetic recording media, the Co alloy thin film should have uniform small grains with grain boundaries capable of magnetically isolating neighboring grains. This type of microstructural and crystallographic control is typically attempted by manipulating the deposition process, grooving the substrate surface and proper use of an underlayer.
Underlayers can strongly influence the crystallographic orientation, the grain size and chemical segregation of the Co alloy grain boundaries. Conventional underlayers include Cr and alloys of Cr with elements such as titanium (Ti), tungsten (W), molybdenum (Mo) and vanadium (V).
There are other basic characteristics of magnetic recording media, aside from SMNR, which are indicative of recording performance, such as half-amplitude pulse width (PW50), overwrite (OW), and modulation level. At high linear recording density, adjacent bits are crowded together. A wide PW50 results in interference which limits the linear packing density of bits in a even track and, hence, reduces packing density in a given area thereby eliminating the recording capacity of the magnetic recording medium. Accordingly, a narrow PW50 is desirable for high areal recording density.
OW is a measure of the ability of the magnetic recording medium to accommodate overwriting of existing data. In other words, OW is a measure of what remains of a first signal after a second signal, e.g., at a different frequency, has been written over it on the medium. OW is considered low or poor when a significant amount of the first signal remains.
It is extremely difficult to obtain optimum performance from a magnetic recording medium by optimizing each of the PW50, OW, SMNR and modulation level, as these performance criteria are interrelated and tend to be offsetting. For example, if coercivity is increased to obtain a narrower PW50, OW is typically adversely impacted, as increasing coercivity typically degrades OW. A thinner medium having a lower Mr x thickness (Mrt) yields a narrower PW50and better OW; however, the medium signal is typically reduced. Increasing the squareness of the hysteresis loop contributes to a narrower PW50, and better OW; however, noise may increase due to intergranular exchange coupling and magnetostatic interaction. Thus, a formidable challenge is present in optimizing magnetic performance in terms of PW50,, OW, SMNR and modulation level.
It is recognized that the magnetic properties, such as Hr, Mr, S* and SMNR, which are critical to the performance of a magnetic alloy film, depend primarily upon the microstructure of the magnetic layer which, in turn, is influenced by the underlying layers, such as the underlayer. It is also recognized that underlayers having a fine grain structure are highly desirable, particular for growing fine grains of HCP Co alloys deposited thereon.
It has been reported that nickel-aluminum (NiAl) films exhibit a grain size which is smaller than similarly deposited Cr films, which are the underlayer of choice in conventional magnetic recording media. Li-Lien Lee et al., xe2x80x9cNiAl Underlayers for CoCrTa Magnetic Thin Filmsxe2x80x9d, IEEE Transactions on Magnetics, Vol. 30, No. 6, pp. 3951-3953, 1994. Accordingly, NiAl thin films are potential candidates as underlayers for magnetic recording media for high density longitudinal magnetic recording. However, it was found that the coercivity of a magnetic recording medium comprising an NiAl underlayer is too low for high density recording, e.g. about 2,000 Oersteds (Oe). The use of an NiAl underlayer is also disclosed by C.A. Ross et al., xe2x80x9cThe Role of An NiAl Underlayers In Longitudinal Thin Film Mediaxe2x80x9d, J. Appl. Phys. 81(8), P.4369, 1997. NiAl underlayers are also disclosed by Lee et al. in U.S. Pat. No. 5,693,426 and Lee et al. in U.S. Pat. No. 5,800,931. A magnetic recording medium comprising a NiAl seedlayer under a Cr underlayer is disclosed by Zhang in U.S. Pat. No. 5, 858,566.
In copending U.S. patent application Ser. No. 09/152,326 filed on Sep. 14, 1998 now U.S. Pat. No. 6,117,570 issued Sep. 12, 2000 a magnetic recording medium is disclosed comprising a NiAl seedlayer having an oxidized surface, a chromium underlayer on the seedlayer, and a magnetic layer on the underlayer. Chen et al. in U.S. Pat. No. 5,153,044 disclose magnetic recording medium comprising a non-magnetic substrate, a plated nickel-phosphorous layer thereon, and a sputtered underlayer comprising a nickel phosphorous layer formed on the plated nickel phosphorous layer. It is further disclosed that the sputtering target can comprise aluminum. Ranjan et al. in U.S. Pat. No. 5,642,094 disclose a magnetic recording medium comprising a plated nickel phosphorous layer on a substrate and an amorphous sputtered nucleation layer of nickel phosphorous deposited on the plated nickel phosphorous layer. The sputtered nucleation layer can comprise alumina. Chen et al. in U.S. Pat. No. 5,851,688 disclose a magnetic recording medium comprising a nickel phosphorous underlayer and a nickel phosphorous nucleation layer sputtered thereon. The nucleation layer can comprise an oxide dopant, such as alumina.
Okumura et al. in U.S. Pat. No. 5,700,593 disclose a magnetic recording medium comprising a substrate and a seedlayer comprising an oxygen-containing non-magnetic amorphous metal or a seedlayer comprising a non-magnetic amorphous metal having an oxygen-containing layer thereon, and an underlying non-magnetic layer laminated on the seedlayer. Doerner et al. in U.S. Pat. No. 5,302,434 disclose a magnetic recording medium comprising an untextured nickel phosphorous coating on a disk substrate which is oxidized to form a nickel oxide film. Suzuki et al. in U.S. Pat. No. 5,587,235 disclose a magnetic recording medium comprising a multi-layer structure containing at least one paramagnetic intermediate region or oxygen-rich region disposed between magnetic layers. Chen et al. in U.S. Pat. No. 5,866,227 disclose a magnetic recording medium comprising a glass or glass-ceramic substrate formed by sequentially depositing thereon a partially oxidized nickel phosphorous seedlayer, an underlayer and a magnetic layer.
There exist a need for high areal density longitudinal magnetic recording media exhibiting high Hr and high SMNR, and methodology for manufacturing such magnetic recording media. There also exist a need for magnetic recording media containing a glass or glass-ceramic substrate exhibiting high Hr and high SMNR.
An advantage of the present invention is a magnetic recording medium for high areal recording density exhibiting low noise and high Hr.
Another advantage of the present invention is a method of manufacturing a magnetic recording medium suitable for high areal recording density and exhibiting low noise and high Hr.
Additional advantages and features of the present invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following only to be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained and particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are achieved by a magnetic recording medium comprising a non-magnetic substrate; a nickel aluminum (NiAl) sub-seedlayer having an oxidized surface on the non-magnetic substrate; a NiAl seedlayer on the surface oxidized NiAl sub-seedlayer; an underlayer on the NiAl seedlayer; and a magnetic layer on the underlayer.
Another aspect of the present invention is a method of manufacturing a magnetic recording medium, the method comprising depositing a nickel aluminum (NiAl) sub-seedlayer on a non-magnetic substrate; oxidizing the surface of the NiAl sub-seedlayer; depositing a NiAl seedlayer on the surface oxidized NiAl sub-seedlayer; and depositing a magnetic layer on the seedlayer.
Embodiments of the present invention comprise sputter depositing a NiAl sub-seedlayer on a glass or glass-ceramic substrate, surface oxidizing the NiAl seedlayer in an oxidizing atmosphere containing about 1 to about 100 vol. % oxygen, e.g., about 20 vol. % oxygen with an inert gas, such as argon, sputter depositing an NiAl seedlayer on the oxidized surface of the NiAl sub-seedlayer, depositing a Cr alloy underlayer, such as CrMo, on the NiAl seedlayer, optionally depositing a flash cobalt- chromium-tantalum (CoCrTa) on the underlayer and depositing a cobalt-chromium magnetic alloy layer on the flash CoCrTa layer. Embodiments include forming the sub-seedlayer at a thickness of about 50 xc3x85 to about 1,000 xc3x85e.g., about 100 xc3x85 to about 500 xc3x85.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its details are capable of modifications in various obvious respects, all without departing from the present invention. Accordingly,, the drawings and description are to be regarded as illustrative in nature and not as restrictive.